In prparing peptides by the interaction of the free amino group of one amino acid or peptide fragment with the free carboxyl group of another amino acid or peptide fragment, it is generally highly preferred that only those reactive functions which are intended to participate in the peptide formation be available during reaction. In other words, any other reactive groups which may be present in the reactants generally will be protected by prior reaction with an appropriate blocking group. Two such reactive functions which are sometimes present in amino acids or peptide fragments and which generally are protected prior to peptide chain building are the hydroxyl function and the thiol function. It has been found that protection of such groups can be achieved by reacting these functions with a suitable reagent to form the tertiary butyl ether or tertiary butyl thioether, respectively.
It is also quite possible that one would wish to protect a carboxyl group of an amino acid or peptide fragment. A tertiary butyl function provides a highly suitable protecting group for this purpose, producing the corresponding tertiary butyl ester. The tertiary butyl group is readily cleavable to re-form the free carboxyl group.
Literature describes the O-t-butylation of sterols using isobutylene when carried out in the presence of a catalyst comprising boron trifluoride etherate and anhydrous phosphoric acid, see H. C. Beyerman and G. J. Heiszwold, Rec. Trav. Chim., 84, 203 (1965). In this publication the use of the catalyst was directed exclusively to O-t-butylation of sterols. No indication or direction is contained in this publication with respect to the t-butylation of hydroxyl groups, thiol groups, or carboxyl groups present in amino acids.
The customary method by which an amino acid or peptide fragment has been esterified, etherified, or thioetherified using isobutylene has been by reacting the acid or peptide fragment with isobutylene in the presence of sulfuric acid as catalyst. Using a sulfuric acid catalyst system, it has been found to be necessary to carry out the reaction over an extended period of time, sometimes lasting even for several days. Since the reaction takes such a long period of time, it is furthermore essential, in order to retain the presence of the isobutylene reactant, to carry out the reaction in a pressure bottle. Moreover, typically the reaction does not proceed to completion. For example, using L-serine as amino acid, in which it is intended that the amino acid be both esterified at the carboxyl group and etherified at the hydroxyl group, it is found that a large portion of the product which results is the t-butyl ester of the amino acid containing intact a free hydroxyl function. That is to say, the conditions of reaction are such that it is possible to esterify the amino acid but not to accomplish substantial etherification. Thus, the sulfuric acid catalyzed t-butylation of hydroxy or thiol amino acids, although suitable for achieving some etherification, nevertheless, exhibits serious limitations.